Share more init work

[Rich-Harris]

Change suggested by @tomcon — we can move more initialisation work into shared functions. The reason it's the way it is currently is to avoid adding extra code for components that don't need it, but realistically this just makes the typically app larger for the sake of sometimes making standalone components smaller.

Taking the realworld app as an example, this PR reduces the total JS size to 157,336 bytes from 160,200, a 1.8% reduction. (Granted, after zipping the difference is almost zilch — 34141 from 34145 — but that's after concatenating all the code-split chunks, and in any case minimising the JS that needs to be parsed and eval'd is the larger goal.)

Compiling a standalone <h1>hello {name}</h1> component goes up from 2,391 to 2,469 bytes, a 3.3% increase. That's a more 'visible' increase for someone evaluating Svelte for the first time, but maybe that's ok?

[ekhaled]

Please also check HMR works with this change.
I have a feeling it still will. But if it doesn't, maybe I can prepare the helper for this change

[Rich-Harris]

Tried it out in sapper-template — HMR looks good 👍